Metachromatic leukodystrophy (MLD) is caused by an autosomal recessive genetic defect in the lysosomal enzyme arylsulfatase A (ASA or ARSA), resulting in a progressive breakdown of membranes of the myelin sheath (demyelination) and accumulation of galactosyl sulphatide (cerebroside sulfate) in the white matter of both central nervous system (CNS) and peripheral nervous system. Patients with metachromatic leukodystrophy can be treated with enzyme replacement therapy (ERT) using recombinant arylsulfatase A (rASA).